Transparent conducting tin oxide films were prepared by a chemical vapour deposition technique. The films were deposited at substrate temperatures of 400, 450 and 500 degrees C. The films were annealed (above the deposition temperatures) up to 1000 degrees C in various gas atmospheres (argon, oxygen, nitrogen and air). It is observed that there is an initial decrease in resistivity up to a temperature of 700 degrees C which is followed by an increase in resistivity beyond 700 degrees C. X-ray and electron diffraction studies have been carried out on the annealed films and the behaviour of this decrease and increase in resistivity has been explained.